This invention relates to view finders for single lens reflex cameras, and more particularly to a view finder using an eye-piece with a plastic lens.
Recently, a strong demand has developed for single lens reflex cameras with viewfinders having a viewfield image of good quality and high finder magnification. On the other hand, the amateur camera is, because of its low price, required to be produced at low cost.
An increase in finder magnification can be achieved by decreasing the size of the roof type pentagonal prism (referred to) to shorten the distance between the focusing plane and the primary principal point of the eye-piece. But, the employment of this arrangement imparts a wider angle to the eye-piece which makes it more difficult to correct astigmatism and distortion.
Decreasing the production cost is most effectively fulfilled by manufacturing the lenses from molded plastic. However, plastics such as acrylic resin have a refractive index which is low as compared to glass materials usually used in the eye-piece. This makes it difficult to correct spherical aberration and distortion. Plastic lenses are also very susceptible to scratches.